This invention relates to a control system for controlling the delivery of three-phase power to a load in order to regulate the operation thereof. While the invention may be employed in any three-phase electrical system where it is desired to control the application to a load of three phase-displaced alternating voltages, it is particularly attractive in controlling the operation of a three-phase motor. Moreover, the invention is especially useful in an air-conditioning system for modulating the speed of a three-phase fan motor for an air-cooled condenser coil in response to the temperature of the refrigerant in the condenser coil, the speed varying directly with temperature, in order to maintain a substantially constant condenser pressure despite wide variations in condenser cooling air temperature. Accordingly, the invention will be described in the environment of an all-weather air-conditioning system which is required to operate in the presence of a broad range of outside ambient temperatures.
The condenser coil of an air-conditioning system is usually located out-of-doors or in heat exchange relation with outdoor air and is therefore subjected to widely varying ambient temperatures. If the system operates during cold weather, the outdoor temperatures may drop sufficiently low to materially reduce the condensing temperature of the refrigerant in the condenser coil. This produces a corresponding reduction in head pressure on the high pressure side of the refrigeration system, resulting in a decreased pressure differential across the thermal expansion valve or other refrigerant metering device in the system. Because of the reduced pressure difference across the metering device, less refrigerant flows from the condenser to the evaporator. The capacity of the refrigeration system is accordingly reduced and the cooling load placed on the evaporator may not be satisfied.
In some instances, the reduction in head pressure at low ambient temperatures may result in the evaporator coil being cooled to a temperature below freezing, allowing condensed moisture to freeze on the evaporator coil. As the layer of ice builds up on the evaporator coil, the coil becomes insulated from the refrigeration load and a further reduction in system capacity occurs.
Systems have been developed for preventing a pressure drop on the high pressure side of the refrigeration system, thereby to maintain the minimum pressure differential across the metering device required for efficient operation, by reducing the speed of at least one fan motor for the condenser as the ambient temperature falls. The volume of air blown across the condenser coil therefore decreases and this limits the amount of heat that can be extracted from the refrigerant as it passes through the condenser coil, insuring that the refrigerant temperature, and consequently its pressure, does not fall below the required minimum. With the pressure on the high side of the system at or above the minimum, the pressure difference across the expansion or metering device will be at or above the level necessary for efficient operation.
The present invention also maintains a minimum head pressure by keying the speed of a three-phase fan motor to condensing temperature. The motor control functions are achieved by means of a control system considerably simpler, more reliable, compact, and less expensive than those developed heretofore. Moreover, the present control system exhibits a significant improvement in performance over the prior systems. Furthermore, it is immune to instabilities due to application problems such as noise, phase rotation, power factor, line voltage, phase unbalance, line transients and line frequency.